When exposed to high temperatures (i.e., greater than or equal to about 1,300° C.) and to oxidative environments, metals can oxidize, corrode, and become brittle. These environments are produced in turbines such as those used for power generation applications. Metallic coatings, when applied to metal turbine components such as via thermal spraying techniques, can reduce the effects that high-temperature, and corrosive and oxidative environments have on the metal components.
The family of thermal spray processes include detonation gun deposition, high velocity oxy-fuel deposition (HVOF) and its variants such as high velocity air-fuel, plasma spray, flame spray, and electric wire arc spray. In most thermal coating processes a material is heated to near or somewhat above its melting point and droplets of the material accelerated in a gas stream. The droplets are directed against the surface of a substrate to be coated where they adhere and flow into thin lamellar particles called splats.
Thermal spray coating processes have been used for many years to deposit layered coatings. These coatings consist of discrete layers of different composition and properties. For example, the coating may be a simple duplex coating consisting of a layer of a metal alloy such as nickel-chromium adjacent to the substrate with a layer of zirconia over it.
A current problem exists when MCrAlY coatings are used in integrated gasification combined cycle (IGCC) systems; that is, systems using an innovative process that uses coal to produce power. The process is cleaner and more economically efficient than other processes that use coal to produce power. The process involves treating coal and reforming coal to a gas mixture that includes hydrogen gas (H2), carbon monoxide (CO), and carbon particulates. This gas mixture is combusted with oxygen in a turbine to produce power. The carbon particulates, however, collide with the coated turbine components and erode the components and/or coatings, and thereby shorten the effective operating life of the components.
Another turbine component problem exists in that they also experience premature failure due to environmental attack, particularly in the hot gas path of the gas turbine engine.
Therefore, there exists a need for articles, such as turbine components, that have enhance resistance to harsh environments such as those in a gas turbine.